Galactic and extragalactic magnetic fields – a concise review

Abstract

The strength of the total magnetic field in our Milky Way from radio synchrotron measurements is about 6µG (0.6 nT), averaged over a radius of about 1 kpc around the Sun. Diffuse polarized radio emission and Faraday rota- tion of the polarized emission from pulsars and background sources show many small-scale magnetic features, but the overall field structure in our Galaxy is still under debate. - In nearby galaxies, radio synchrotron observations re- veal dynamically important magnetic fields of 10 30µG (1 3 nT) total strength in the spiral arms. Fields with ran- dom orientations are concentrated in spiral arms, while or- dered fields (observed in radio polarization) are strongest in interarm regions and follow the orientation of the adjacent gas spiral arms. Faraday rotation of the diffuse polarized radio emission from the disks of spiral galaxies sometimes reveals large-scale patterns which are signatures of regular fields generated by dynamos, but in most galaxies the field structure is more complicated. - Strong magnetic fields are also observed in radio halos around edge-on galaxies, out to large distances from the plane. The ordered halo fields usually form an X-shaped pattern. Diffuse polarized radio emission in the outer disks and halos is an excellent tracer of galaxy interactions and ram pressure by the intergalactic medium. from starburst galaxies in the early Universe may have mag- netized the intergalactic medium. In spite of our increasing knowledge of magnetic fields, many important questions, especially the origin and evolu- tion of magnetic fields, their first occurrence in young galax- ies and the existence of large-scale intergalactic fields remain unanswered. Furthermore, the detection of ultrahigh-energy cosmic rays with the AUGER observatory calls for a detailed knowledge of the magnetic field in the Milky Way to model particle propagation.